1. Field of the Invention
The present invention relates, in general, to a phase shift mask and, more particularly, to a phase shift mask which is a vernier or box-in-box pattern type such that the degree of overlap between the light screen pattern and the phase shift film pattern can be examined accurately, thereby improving product yield and reliability.
2. Description of the Prior Art
Following the recent trend in semiconductor devices to be simple, thin, light and compact, the size of unit devices, such as transistors and capacitors, should be reduced along with the distance between wires. As topology in semiconductor device increases, the fineness of pattern is accelerated.
A typical exposure mask that is used at an exposing step to form a photosensitive film pattern is prepared by coating a light screen film, such as a chrome or an aluminum layer, on a substrate and patterning the light screen film by means of ion beam etching. However, it is difficult to form a fine pattern smaller than light resolution by use of the typical mask. Indeed, it is virtually .impossible to obtain a fine pattern with a width of .mu.m or less by use of current photosensitive solutions and light exposing equipment, for example, G line (436 nm) or I line (365 nm) steppers.
In addition, very highly integrated semiconductor devices, such as 64M DRAM, demand a fine pattern with a width smaller than 0.5 .mu.m. Various efforts have been made to meet such a demand. In an effort to develop highly integrated semiconductor devices, phase shift masks were invented. In fact, the ultrafine pattern is accomplished by use of phase shift masks.
A phase shift mask is essentially composed broadly of a light screen pattern and a phase shift film pattern. The phase shift film pattern plays the role of shifting an incident beam at an angle of 180.degree. or 90.degree.. Such a phase shift mask is designed to keep the amplitude of the light illuminated on the wafer constant at the exposing step and to minimize the exposure effect caused by interference between a beam passing through one line of the phase shift pattern and another beam passing through another line adjacent to the one line, thereby improving the resolution of the photosensitive film pattern.
In order to improve the contrast ratio of a light illuminated on a photosensitive film, the phase shift film is so thick as to shift the phase of the light at an angle of 160.degree. to 200.degree.. For example, where an incident beam is of G or I line and the phase shift material is spin-on-glass (hereinafter referred to as "SOG"), an oxide or a nitride, the phase shift film has a thickness ranging from 3,400 to 4,000 Angstrom. Such a phase shift mask allows the lines of the pattern to have a width of 0.5 .mu.m or less even when conventional photosensitive films and light exposing equipments are employed.
Now, in order to better understand the background of the present invention, a description of conventional repetitive phase shift masks will be presented in connection with FIGS. 1 to 3.
First, referring to FIG. 1, there is shown a conventional example or a space frequency-modulating phase shift mask. As shown in this figure, a plurality of light screen film patterns 2 is formed on a quartz substrate 1. The chrome patterns are arranged so that each of them has a predetermined width and are uniformly separated. A plurality of phase shift film patterns 3 is also formed on every other space between the light screen patterns 2.
FIG. 2 shows another example of a space frequency-modulating phase shift mask. As shown in FIG. 2, a phase shift film pattern 3 is formed on a quartz substrate 1, A plurality of light screen film patterns 2 is formed on the phase shift film pattern, arranged so that each of them has a predetermined width and is uniformly separated. Every other space of the phase shift film pattern's areas corresponding to the space between the light screen film patterns may be removed. Thus, the quartz substrate 1 is correspondingly exposed.
Referring to FIG. 3, there is also another example of a phase shift mask. It is designed to Shift the phase of light in accordance with the thickness of the quartz substrate by removing to a predetermined depth every other part of the quartz substrate 1 corresponding to the space between light screen film patterns.
In a phase shift mask, the light screen film pattern is generally formed so as to overlap with the phase shift film patterns to some degree. Since the degree of overlap is a very important factor that affects the performance of phase shift mask, for example, resolution and depth of focus it is important to ascertain the accuracy of the degree of overlap prior to subsequent wafer processing.
Without separate equipment, the accuracy of the overlap degree is difficult to ascertain in conventional phase shift mask structures because the light passing through the quartz substrate and the light passing through the phase shift film pattern both have a transmission of almost one.